AM79C978VCSLASHW datasheet(66/261 Pages) AMD

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Am79C978

dering and content in each of these fields in the frame.

The MAC does not use the content in the length/type

field unless APAD_XMT (CSR4, bit 11) is set and the

data portion of the frame is shorter than 60 bytes.

The MAC engine will detect the incoming preamble se-

quence when the RX_DV signal is activated by the in-

ternal PHY. The MAC will discard the preamble and

begin searching for the SFD. Once the SFD is de-

tected, all subsequent nibbles are treated as part of the

frame. The MAC engine will inspect the length field to

ensure minimum frame size, strip unnecessary pad

characters (if enabled), and pass the remaining bytes

through the receive FIFO to the host. If pad stripping is

performed, the MAC engine will also strip the received

FCS bytes, although normal FCS computation and

checking will occur. Note that apart from pad stripping,

the frame will be passed unmodified to the host. If the

length field has a value of 46 or greater, all frame bytes

including FCS will be passed unmodified to the receive

buffer, regardless of the actual frame length.

If the frame terminates or suffers a collision before 64

bytes of information (after SFD) have been received,

the MAC engine will automatically delete the frame

from the receive FIFO, without host intervention.

TheAm79C978 controller has the ability to accept runt

packets for diagnostic purposes and proprietary net-

works.

Destination Address Handling

The first 6 bytes of information after SFD will be inter-

preted as the destination address field. The MAC en-

gine provides facilities for physical (unicast), logical

(multicast), and broadcast address reception.

Error Detection

The MAC engine provides several facilities which re-

port and recover from errors on the medium. In addi-

tion, it protects the network from gross errors due to

inability of the host to keep pace with the MAC engine

activity.

On completion of transmission, the following transmit

status is available in the appropriate Transmit Message

Descriptor (TMD) and Control and Status Register

(CSR) areas:

n The number of transmission retry attempts (ONE,

MORE, RTRY, and TRC).

n Whether the MAC engine had to Defer (DEF) due to

channel activity.

n Excessive deferral (EXDEF), indicating that the

transmitter experienced Excessive Deferral on this

transmit frame, where Excessive Deferral is defined

in the ISO 8802-3 (IEEE/ANSI 802.3) standard.

n Loss of Carrier (LCAR), indicating that there was an

interruption in the ability of the MAC engine to mon-

itor its own transmission. Repeated LCAR errors in-

dicate a potentially faulty transceiver or network

connection.

n Late Collision (LCOL) indicates that the transmis-

sion suffered a collision after the slot time. This is in-

dicative of a badly configured network. Late

collisions should not occur in a normal operating

network.

n Collision Error (CERR) indicates that the trans-

ceiver did not respond with an SQE Test message

within the first 4 ms after a transmission was com-

pleted. This may be due to a failed transceiver, dis-

connected or faulty transceiver drop cable, or

because the transceiver does not support this fea-

ture (or it is disabled). SQE Test is only valid for 10-

Mbps networks.

In addition to the reporting of network errors, the MAC

engine will also attempt to prevent the creation of any

network error due to the inability of the host to service

the MAC engine. During transmission, if the host fails

to keep the transmit FIFO filled sufficiently, causing an

underflow, the MAC engine will guarantee the message

is either sent as a runt packet (which will be deleted by

the receiving station) or as an invalid FCS (which will

also cause the receiver to reject the message).

The status of each receive message is available in the

appropriate Receive Message Descriptor (RMD) and

CSR areas. All received frames are passed to the host

regardless of any error. The FRAM error will only be re-

ported if an FCS error is detected and there is a non-

integral number of bytes in the message.

During the reception, the FCS is generated on every

nibble (including the dribbling bits) coming from the ca-

ble, although the internally saved FCS value is only up-

dated on the eighth bit (on each byte boundary). The

MAC engine will ignore up to 7 additional bits at the end

of a message (dribbling bits), which can occur under

normal network operating conditions. The framing error

is reported to the user as follows:

n If the number of dribbling bits are 1 to 7 and there is

no FCS error, then there is no Framing error

(FRAM = 0).

n If the number of dribbling bits are 1 to 7 and there is

a FCS error, then there is also a Framing error

(FRAM = 1).

n If the number of dribbling bits is 0, then there is no

Framing error. There may or may not be a FCS er-

ror.

n If the number of dribbling bits is 8, then there is no

Framing error. FCS error will be reported, and the

receive message count will indicate one extra byte.

Counters are provided to report the Receive Collision

Count and Runt Packet Count for network statistics

and utilization calculations.

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